For electron tube parts, especially for the electrodes of electron guns for color picture tubes, nonmagnetic stainless steels have hitherto been used. The nonmagnetic stainless steels, with a certain extent of formability for both deep drawing and burring, have not posed major problems when used for the electrodes of electron guns for conventional-color picture tubes.
The term "burring" as used herein means a working technique whereby a round hole is made in sheet metal while forming a burr or flange protruding from the periphery of the hole. Burring is in wide use with holes for internal threading, bearing, reinforcement, and other purposes. More recently, the advent of higher refinement color picture tubes has made it necessary to increase the lens aperture diameter of the electrodes while performing burring with greater precision and forming higher or taller burrs (the burring for this purpose being called "high burring") so as to improve the focusing characteristics of the electron guns. High burring is required to ensure greater stabilization of the lens focusing characteristics.
Generally, the better the deep drawability and the higher the Lankford value of a material the lower the height of the burrs that can be formed on that material. To increase the height of the burrs, it is to practice to give a better finish to the edges of the holes or reduce the percentage of the inclusions in a material that can cause cracking. Other approaches include the use of a material that has been pickled after a heat treatment called 2D in final annealing or surface polishing of a finally annealed material for improved lubricity with respect to a die.
These approaches are effective to some degree in decreasing the frequency of occurrence of cracks due to burring. However, such decrease of crack occurrence frequency is not satisfactory, especially with the electrodes of electron guns where the burr height is more than one-third of the hole diameter. Another disadvantage is that the need of conditioning the hole ends or surface requires an additional number of process steps.